I believe it is not how to match email address, but how to match the whole RFC822 address which includes one or more email address. An email address looks like this: john@example.com. But an RFC822 address may look something like “John Doe” <john@example.com>.

There are lots of valid email addresses that typical regexps don’t catch. IIRC, the following is a valid email address: “a b c”@[127.0.0.1] – a regexp is really not enough to validate an email address, because you end up with a monstrosity like this (with all respect to the author).